Colorectal cancer is the third most common cancer in the US, with approximately 145,000 new cases expected in 2005. Estimated 5-year survival rates range from 90% for patients with early diagnosis (stage I disease) and fall to <10% for patients with metastatic colorectal cancer (CRC). In this renewal application, we move to studies of the role of Reg IV in the development of mucosal cancers. Studies during the current funding period, presented as the foundation for this application, have demonstrated that: i) Expression of Reg IV is increased in adenomatous polyps and in CRCs, often to very high levels, ii) Increased expression Reg IV is one of the earliest steps in the formation of tumors, coinciding with the second spontaneous mutation in the APC gene and adenomatous change by histology in APCmin/+mice, iii) Reg IV is a potent activator of a signaling pathway (the EGFR-Akt-AP1) in CRC and leads to increased expression of Bcl-2, Bcl-XL, survivin and matrilysin. These genes have all previously been associated with a poor prognosis in advanced CRC, iv) Treatment of CRC cells with recombinant Reg IV protein significantly reduces their susceptibility to death by programed cell death (apoptosis), following radiation or chemotherapeutic drugs, and v) Function blocking anti-Reg IV mAbs or interfering RNAs have potent antagonistic effects on cell growth and increase susceptibility to apoptotic death following radiation or administration of chemotherapeutic drugs. By utilizing the approaches, outlined in the proposal, our goal is to better understand the mechanisms involved in the clinical resistance of CRC to conventional therapy and to ascertain the role of Reg IV as a cause of poor outcome. These studies will also provide new insight into the role of Reg IV antagonists and lay the groundwork for development of therapeutic reagents active in gastrointestinal adenocarcinomas.